There are many products intended for application to the skin for therapeutic or cosmetic purposes that are best formulated as anhydrous systems. Anhydrous liquids or semi-solids were the original topical ointment and lotion vehicles. Mineral oil and petrolatum are examples, as are the animal and vegetable fats and oils, of the bases used as dermatological vehicles or cosmetic carriers. Pharmaceutical formularies include ointments, creams and lotions, containing examples of anhydrous dermatological vehicles for topical application to the skin.
Although these anhydrous topical bases were excellent delivery systems for medications and were proven to be the best skin protective and moisturizing agents, their use is limited by the fact that consumers reject them on the basis of their greasy feel. This essential negative has limited their application to products for serious therapeutic and cosmetic uses, where their greasiness is tolerated.
There have been several attempts to make dermatological vehicles more aesthetically acceptable. In one such attempt clays, starches, polysaccharides or cellulosics have been used to replace the oils in anhydrous systems in order to dry them out. These substitutions have generally been without success. While the substitute materials can be used in small amounts they are usually not successful unless used in large amounts, where these particular materials cause the products to become pasty and sticky and to cause agglomeration of settling materials or clogging in aerosols.
In a further effort to improve dermatological vehicles, water soluble and water dispersible (washable) systems were developed. The majority of these were oil-in-water emulsion systems in which the oil was dispersed in at least 50% water to form a lotion or a cream. These creams spread easily on the skin, were rubbed in, or were absorbed rapidly with reduced greasiness or stickiness and are much more cosmetically elegant and acceptable to consumers.
Although these systems have thus been considered superior to anhydrous topical bases, they suffer from some negatives as compared to the original anhydrous systems.
Newer washable systems are more easily removed so their action is fleeting.
They are perceived as wet and cold feeling during application and require time for rub-in or absorption to feel dry.
The emulsifier in these systems used are sometimes irritating to the skin.
The protective, emollient or moisturizing effect of the washable systems is very significantly less than the anhydrous systems.
Water sometimes results in active ingredient instability; as with antibiotics.
Water presents microbiological problems requiring the use of preservatives.
Emulsions are inherently unstable systems that separate in time.